For known spark plugs having a planar seal seat there is usually provided a sealing ring which forms a sealing between the spark plug and the cylinder head so that the overpressure generated in in the cylinder does not escape through the spark plug bore. Every once in a while, spark plugs are unscrewed from the cylinder head for maintenance purposes. In order that the sealing ring does not slip off from the external thread of the spark plug and get lost, the spark plug is usually provided with a self-retaining sealing ring. The sealing ring engages in a thread undercut arranged between the external thread and a collar of the spark plug in such a manner that it cannot slip off over the external thread of the spark plug by itself. Such a sealing ring is sometimes also designated as a “captive sealing ring.”
Solid sealing rings for spark plugs, which, after sliding the sealing ring onto the external thread of the spark plug are deformed by means of an embossing or caulking process in such a manner that the sealing ring can no longer slip back over the external thread, are known in practice from many applications. The deformation of the sealing ring resulting from the embossing process forms a positive-locking fit with the external thread or the thread undercut and thereby ensures captivity. During the embossing or caulking process, relatively high forces are applied to the otherwise finished spark plug, which can have a negative effect. Moreover, the embossing or caulking process can result in deformations on the sealing surface of the sealing ring, which can impair the sealing quality of the spark plug when mounted later in the cylinder head.
Sealing rings having a solid annular sealing element made from metal have the advantage that they allow accurate positioning of the spark plug screwed into the cylinder head. These sealing rings are therefore primarily used if accurate positioning of the spark position of the spark plug is required, and hence if a predetermined protrusion dimension is to be met precisely with which electrodes, which form a spark gap at the front end of the spark plug, protrude into the combustion chamber.
Spark plugs are often also sealed with non-generic sealing rings which are made without a solid sealing element from a sheet metal strip that is folded multiple times in a U-shaped manner. Such folded sheet metal sealing rings can be deformed using low forces and thus can be easily attached to the spark plug in a self-retaining or captive manner. However, these non-solid sealing rings also deform easily and to a relatively large extent when the spark plug is screwed into the cylinder head and is tightened with a predetermined torque. When tightening the spark plug, a significant plastic deformation of the sealing ring occurs so that neither a predefined circumferential position of a ground electrode nor a predetermined protrusion dimension nor a predetermined spark position can be achieved. As an improvement, DE 600 13 014 T2 and EP 1 508 947 A1 each disclose a sealing ring which can be deformed with little force over a wide range so that when screwing in, the spark plug can be turned to such an extent that the ground electrode thereof is situated in a predefined circumferential position. However, in doing so, the screw-in depth of the spark plug is variable to a high degree resulting in great deviations for the protrusion dimension of the electrodes.
In DE 10 2011 015 183 A1, a non-generic spark plug without a sealing ring is disclosed, wherein the external thread is surrounded at least on a portion of its length by a sealing agent for sealing the combustion chamber. Although such a spark plug cannot lose a sealing ring, there is, however, the danger that the sealing of the spark plug is no longer ensured after unscrewing the spark plug from the cylinder head and subsequently screwing in the spark plug again.
It may be an object of the invention to provide a sealing ring for a spark plug, a spark plug and an internal combustion engine by means of which a predefined spark position in the combustion chamber and a predefined protrusion dimension of the spark plug electrodes protruding into the combustion chamber can be achieved in a precise manner.